Wrongly held beliefs about wrongly held beliefs

An interesting question was posed over at the edge: http://bit.ly/fh8wij 

The flat earth and geocentric world are examples of wrong scientific beliefs that were held for long periods. Can you name your favorite example and for extra credit why it was believed to be true?

A number of interesting responses popped up, but, thankfully, someone did in fact point out the wrongly held belief in the question itself: 

I am of course aware of the currently popular belief that "flat earth" was somehow a widely held "scientific" idea, but I do not know what evidence supports this belief. It was certainly not part of the Antique inheritance (who had pretty good estimates for the diameter of the earth and excellent estimates for the ratio of Earth's and Moon's diameters); It was not part of Aristotle, or Aquinus, or any of the authorities that the Church relied on. No doubt, there were some creation myths or fanciful publications that might have illustrated the world as being flat but it is a stretch to call these "scientific" even by standards of the age, when learned men would have been able to refute such a thesis easily — and probably did as part of their exams.

Computer Scientist, International Software; Former Chief Architect, and Distinguished Engineer, Microsoft Corporation